I'd start by measuring a standard or two. You can get pretty decent
termination resistors on eBay for a modest cost. If your system measures
close to 1:1 when terminated with one of those, you're off to a good
start. Next I'd make up a 50 ohm microstrip line terminated with a 50
ohm chip resistor. If that reads close to what the 50 ohm termination
did, then I'd measure the SWR with some different load values, say 25
and 100 ohms. These of course could be made from two 50 ohm chip
resistors. You could get more elaborate with the calibration by
introducing reactive loads, but just the three data points should be
enough to give you some confidence in your SWR measurements, at least
near 1:1 where your primary interest is.

Roy Lewallen, W7EL